Chapter 3. Contrivance a Necssity Arising Out of the Reign of Law — Example in thr Machinery of Flight

George Campbell, Duke of Argyll

Credits

John van Wyhe, Ph.D., Cambridge University digitized and converted to html George Campbell's The Reign of Law (London, 1867). George P. Landow proofed, reformatted, and added links to the text in December 2008.

Directions

Clicking upon the superscript numbers bring you to notes in the left — hand column; clicking upon the back button on your browser returns you to your place in the main text.

Page breaks in the original are indicated as follows: [498/499]. Where pagebreaks divide words, the complete word appears after the page break

Notes

3.
The upward stroke has no sustaining power, but has considerable propelling
power; because some air, failing to escape between the feathers, must always
pass along the convex surface of the wing, and escaping backwards, must exert
upon the ends of the quills a similar reactive force to that which is exerted
in the downward stroke.

4. For the form of the wing in this remarkable bird, see the beautiful drawing
here engraved from the pencil of Mr Wolf.

5. The Albatross, when rising from the sea, is described ("Ibis," July 1865)
as"stretching out his neck, and with great exertion of his wings running along
the top of the water for seventy or eighty yards, until at last having got sufficient
impetus, he tucks up his legs, and is once more fairly launched into the air."

7. The mechanical principle involved in the sufficiency of very narrow wings
has, I believe, been adequately explained in a very ingenious Paper read before
the Aeronautical Society, by Mr F. H. Wenham, C. E. It is the same mechanical
principle which accounts for the narrow blades of a Screw Propeller having a
resisting force as great as would be exerted upon the whole area of rotation
by a solid Disc. In the case of a flat body, such as the wing of a Bird, being
propelled edgeways through the air, nearly the whole resisting and sustaining
force is exerted upon the first few inches of the advancing surface.

8. I owe to the accurate pencil of Mr J. Wolf the accompanying engraving of the wing of the Golden Plover, a bird of powerful flight. In this wing the gradation of the feathers is very perfect. It will be observed that the first of the Secondaries, the eleventh feather from the tip of the wing is marked by a slight variation in he form of the margin.

9. The illustrations of Mr Wolf will here again be the best explanation to the
reader of the difference between the sharp and the round structure, p. 166.

10. Mr Wolf's illustration of a Kestrel hovering shows accurately the position of the Bird when the action is performed in still air.

12. See a very interesting account of the flight of the Albatross by Captain T. W. Hutton, in the "Ibis" for July 1864. Captain Hutton says, "If he wishes to turn to the right, he bends his head and tail slightly upwards, at the same time raising his left side and lowering the right in proportion
to the sharpness of the curve he wishes to make, the wings being kept rigid the whole time." This is the only paper I have seen on the flight of birds in
which observation of the facts is not vitiated by some false preconceived theory
on their cause. Captain Hutton has thoroughly seized the true mechanical principles of flight.

13.The men of science in France are ahead of the men of science in England upon
this subject. There is a Society established in Paris which announces in its
very title the true fundamental principle of flight,"Société
d'Encouragement pour la Locomotion Aérienne au moyen d'Appareils PLUS
LOURDS que l'Air." The false principle of Buoyancy is thus eliminated and banished
from the question.

THE necessity of Contrivance for the accomplishment of Purpose arises out of
the immutability of Natural Forces. They must be conformed to, and obeyed.
Therefore, where they do not serve our purpose directly, they can only be made
to serve it by ingenuity and contrivance. This necessity, then, may be said
to be the index and the measure of the power of Law. And so, on the other hand,
the certainty with which Purpose can be accomplished by Contrivance, is the
index and the measure of mental knowledge and resource. It is by wisdom and
knowledge that the Forces of Nature — even those which may seem most adverse — are
yoked to service. This idea of the relation in which Law stands to Will, and
[128/129] in which Will stands to Law, is familiar to us in the works of Man: but it
is less familiar to us as equally holding good in the works of Nature. We feel,
sometimes, as if it were an unworthy notion of the Will which works in Nature,
to suppose that it should never act except through the use of means. But our
notions of unworthiness are themselves often the unworthiest of all. They must
be ruled and disciplined by observation of that which is, — not founded on a priori
conceptions of what ought to be. Nothing is more certain than that the whole
Order of Nature is one vast system of Contrivance. And what is Contrivance but
that kind of arrangement by which the unchangeable demands of Law are met and
satisfied? It may be that all natural Forces are resolvable into some One Force,
and indeed in the modern doctrine of the Correlation of Forces, an idea which
is a near approach to this, has already entered the domain of Science. It may
also be that this One Force, into which all others return again, is itself but
a mode of action of the Divine Will. But we have no instruments whereby to reach
this last analysis. Whatever the [129/130] ultimate relation may be between mental and material Force, we can at least
see clearly this,that in Nature there is the most elaborate machinery to accomplish
Purpose through the instrumentality of means. It seems as if all that is done
in Nature as well as all that is done in art, were done by knowing how to
do it. It is curious how the language of the great Seers of the Old Testament
corresponds with this idea. They uniformly ascribe all the operations of Nature — the
greatest and the smallest — to the working of Divine Power. But they never revolt — as
so many do in these weaker days — from the idea of this Power working by wisdom
and knowledge in the use of means: nor, in this point of view, do they ever
separate between the work of first Creation, and the work which is going on
daily in the existing world. Exactly the same language is applied to the rarest
exertions of power, and to the gentlest and most constant of all natural operations.
Thus the saying that"The Lord by wisdom hath founded the Earth: by understanding
hath He established the Heavens," — is coupled in the same breath with this other
saying, "By His knowledge [130/131] the depths are broken up, and the clouds drop down the dew."1

Every instance of Contrivance which we can thoroughly follow and understand,
has an intense interest — as casting light upon this method of the Divine government,
and upon the analogy between the operations of our own minds and the operations
of the Creator. Some instances will strike us more than others — and those will
strike us most which stand in some near comparison with our own human efforts
of ingenuity and contrivance. There is one such instance which I propose to
consider in this chapter — the machinery by which a great purpose has been accomplished
in Nature — a purpose which Man has never been able to accomplish in art, and
that is the Navigation of the Air. No more beautiful example can be found, even
in the wide and rich domain of Animal Mechanics — none in which we can trace more
clearly, too, the mode and method in which laws the most rigorous and exact,
are used as the supple instruments of Purpose.
[131/132]"The way of an Eagle in the air" was one of the things of which Solomon said,
that "he knew it not." No wonder that the Wise King reckoned it among the great
mysteries of Nature! The Force of Gravitation, though its exact measure was
not ascertained till the days of Newton, has been the most familiar of all Forces
in all ages of Mankind. How, then, in violation of its known effects, could
heavy bodies be supported upon the thin air — and be gifted with the power of
sustaining and directing movements more easy, more rapid, and more certain than
the movements of other animals upon the firm and solid earth? No animal motion
in Nature is so striking or so beautiful as the —

"Scythe — like sweep of wings, that dare
The headlong plunge through eddying gulfs of air."2

Nor will the wonder cease when, so far as the mechanical problem is concerned,
the mystery of flight is solved. If we wish to see how material laws can be
bent to purpose, we shall study this problem.

In the first place it is remarkable that the Force which seems so adverse — the
Force of Gravitation [132/133]
drawing down all bodies to the earth, is the very
Force which is the principal one concerned in flight, and without which flight
would be impossible. It is curious how completely this has been forgotten in
almost all human attempts to navigate the air. Birds are not lighter than the
air, but immensely heavier. If they were lighter than the air they might float,
but they could not fly. This is the difference between a bird and a balloon.
A balloon rises because it is lighter than the air, and floats upon it. Consequently,
it is incapable of being directed, because it possesses in itself no active
Force enabling it to resist the currents of the air in which it is immersed,
and because if it had such a force it would have no fulcrum, or resisting medium
against which to exert it. It becomes, as it were, part of the atmosphere, and
must go with it where it goes. No bird is ever for an instant of time lighter
than the air in which it flies; but being, on the contrary, always greatly heavier,
it keeps possession of a Force capable of supplying momentum, and therefore
capable of overcoming any lesser Force, such as the ordinary resistance of the
atmosphere, and even of heavy gales of wind. [133/134] The Law of Gravitation, therefore, is used in the flight of birds as one of
the most essential of the Forces which are available for the accomplishment
of the end in view.

The next law appealed to, and pressed into the service, is again a law which
would seem an impediment in the way. This is the resisting force of the atmosphere
in opposing any body moving through it. In this force an agent is sought and
found for supplying the requisite balance to the Force of Gravity. But in order
that the resisting force of air should be effectual for this purpose, it must
be used under very peculiar conditions. The resisting force of fluids, and of
airs or gases, is a force acting equally in all directions, unless special means
are taken to give it predominant action in some special direction. If it is
a force strong enough to prevent a body from falling, it is also a force strong
enough to prevent it from advancing. In order, therefore, to solve the problem
of flight, the resisting power of the air must be called into action as strongly
as possible in the direction opposite to the Force of Gravity, and as little
as possible in any other. Consequently a body [134/135] capable of flight must present its maximum of surface to the resistance of
the air in the perpendicular direction, and its minimum of surface in the horizontal
direction. Now, both these conditions are satisfied (1) by the great breadth
or length of surface presented to the air perpendicularly in a bird's expanded
wings, and by (2) the narrow lines presented in its shape horizontally, when
in the act of forward motion through the air. But something more yet is required
for flight. Great as the resisting force of air is, it is not strong enough
to balance the Force of Gravity by its mere pressure on an expanded wing — unless
that pressure is increased by an appeal to yet other laws — and other properties
of its nature. Every sportsman must have seen cases in which a flying bird has
been so wounded as to produce a rigid expansion of the wings. This does not
prevent the bird from falling, although it breaks the fall, and makes it come
more or less gently to the ground.

Yet further, therefore, to accomplish flight, another law must be appealed
to, and that is the immense elasticity of the air, and the reacting force it
exerts against compression. To enable an [135/136] animal heavier than the air to support itself against the Force of Gravity,
it must be enabled to strike the air downwards with such force as to occasion
are bound upwards of corresponding power. The wing of a flying animal must therefore
do something more than barely balance Gravity. It must be able to strike the
air with such violence as to call forth a reaction equally violent, and in the
opposite direction. This is the function assigned to the powerful muscles by
which the wings of birds are flapped with such velocity and strength. We need
not follow this part of the problem further, because it does not differ in kind
from the muscular action of other animals. The connexion, indeed, between the
Wills of animals and the mechanism of their frame is the last and highest problem
of all in the mechanics of Nature, but it is merged and hid for ever in the
one great mystery of Life! But so far as this difficulty is concerned the action
of an Eagle's wing is not more mysterious than the action of a Man's arm. There
is a greater concentration of muscular power in the organism of birds than in
most other animal frames, because it is an essential part of the [136/137] problem to be solved in flight, that the engine which works the wings should be
very strong, very compact, of a special form, and that, though heavier than
the air, it should not have an excessive weight. These conditions are all met
in the power, in the outline, and in the bulk of the pectoral muscles which
move the wings of birds. Few persons have any idea of the force expended in
the action of ordinary flight. The pulsations of the Wing in most birds are
so rapid that they cannot be counted. Even the Heron seldom flaps its wings
at a rate of less than from 120 to 150 strokes in a minute. This is counting
only the downward strokes, preparatory to each one of which there must be an
upward stroke also: so that there are from 240 to 300 separate movements per
minute. Yet the Heron is remarkable for its slow and heavy flight, and it is
difficult to believe, until one has timed the pulsations with a watch, that
they have a rapidity approaching to two in a second. But this difficulty is
an index to the enormous comparative rapidity of the faster — flying birds. Let
any one try to count the pulsations of the wing in ordinary flight of a Pigeon,
or of a Blackcock, or of a [137/138] Partridge, or still more of any of the diving sea-fowl. He will find that though
in the case of most of these birds the quickness of sight enables him to see
the strokes separate from each other, it is utterly impossible to count them;
whilst in some birds, especially in the Divers, as well as in the Pheasant and
Partridge tribe, the velocity is so great that the eye cannot follow it at all,
and the vibration of the wings leaves only a blurred impression on the eye.

Our subject here, however, is not so much the amount of vital force bestowed
on birds, as the mechanical laws which are appealed to in order to make that
force effective in the accomplishment of flight. The elasticity of the air is
the law which offers itself for the counteraction of gravity. But in order to
make it available for this purpose, there must be some great force of downward
blow in order to evoke a corresponding rebound in the opposite, or upward direction.
Now, what is the nature of the implement required for striking this downward
blow? There are many conditions it must fulfil. First, it must be large enough
in area to compress an adequate volume of air; next, it [138/139] must be light enough in substance not to add an excess of weight to the already
heavy body of the bird; next, it must be strong enough in frame to withstand
the pressure which its own action on the air creates. The first of these conditions
is met by an exact adjustment of the size or area of the wing to the size and
weight of the bird which it is to lift. The second and the third conditions
are both met by the provision of a peculiar substance, feathers, which are very
light, and very strong; whilst the only heavy parts of the framework, namely,
the bones in which the feathers are inserted, are limited to a very small part
of the area required.

But there is another difficulty to be overcome a difficulty opposed by natural
laws, and which can only be met by another adjustment, if possible more ingenious
and beautiful than the rest. It is obvious that if a bird is to support itself
by the downward blow of its wings upon the air, it must at the end of each downward
stroke lift the wing upwards again, so as to be ready for the next. But each
upward stroke is in danger of neutralising the effect of the downward stroke.
It must be made with equal velocity, and if it required [139/140] equal force, it must produce equal resistance, — an equal rebound from the elasticity
of the air. If this difficulty were not evaded somehow, flight would be impossible.
But it is evaded by two mechanical contrivances, which, as it were, triumph
over the laws of aërial resistance by conforming to them. One of these
contrivances is that the upper surface of the wing is made convex, whilst the
under surface is concave. The enormous difference which this makes in atmospheric
resistance is familiarly known to us by the difference between the effect of
the wind on an umbrella which is exposed to it on the under or the upper side.
The air which is struck by a concave or hollow surface, is gathered up, and
prevented from escaping, whereas the air struck by a convex or bulging surface
escapes readily on all sides, and comparatively little pressure or resistance
is produced: And so, from the convexity of the upper surface of a bird's wing,
the upward stroke may be made with comparatively trifling injury to the force
gained in the downward blow.

But this is only half of the provision made against a consequence which would
be so fatal to [140/141] the end in view. The other half consists in this that the feathers of a bird's
wing are made to underlap each other, so that in the downward stroke
the pressure of the air closes them upwards against each other, and converts
the whole series of them into one connected membrane, through which there is
no escape; whilst in the upward stroke the same pressure has precisely the
reverse effect — it opens the feathers, separates them from each other, and converts
each pair of feathers into a self — acting valve, through which the air rushes
at every point. Thus the same implement is changed in the fraction of a second
from a close and continuous membrane which is impervious to the air, into a
series of disconnected joints through which the air passes without the least
resistance — the machine being so adjusted that when pressure is required the
maximum of pressure is produced, and when pressure is to be avoided, it is avoided
in spite of rapid and violent action.

This, however, exhausts but a small part of the means by which Law is made
to do the work of Will in the machinery of flight? It might easily be that violent
and rapid blows struck downwards [141/142] against the elastic air, might enable animals possessed of such power to lift
themselves from the ground and nothing more. There is a common toy which lifts
itself in this manner from the force exerted by the air in resisting, and reacting
upon little vanes which are set spinning by the hand. But the toy mounts straight
up, and is incapable of horizontal motion. So, there are many structures of
wing which might enable animals to mount into the air, but which would not enable
them to advance or to direct their flight. How, then, is this essential purpose
gained? Again we find an appeal made to natural laws, and advantage taken of
their certainty and unchangeableness.

The power of forward motion is given to birds, first by the direction in which
the whole wing feathers are set, and next by the structure given to each feather
in itself. The wing feathers are all set backwards, that is, in the direction
opposite to that in which the bird moves, whilst each feather is at the same
time so constructed as to be strong and rigid toward its base, and extremely
flexible and elastic towards its end. On the other hand, the front of the wing,
along the greater part of its [142/143] length, is a stiff hard edge, wholly unelastic and unyielding to the air. The
anterior and posterior webs of each feather are adjusted on the same principle.
The consequence of this disposition of the parts as a whole, and of this construction
of each of the parts, is, that the air which is struck and compressed in the
hollow of the wing, being unable to escape through the wing, owing to
the closing upwards of the feathers against each other, and being also unable
to escape forwards owing to the rigidity of the bones and of the quills in that
direction, finds its easiest escape backwards. In passing backwards it lifts
by its force the elastic ends of the feathers; and thus whilst effecting this
escape, in obedience to the law of action and reaction, it communicates, in
its passage along the whole line of both wings, a corresponding push forwards
to the body of the bird. By this elaborate mechanical contrivance the same volume
of air is made to perform the double duty of yielding pressure enough to sustain
the bird's weight against the Force of Gravity, and also of communicating to
it a forward impulse. The bird, therefore, has nothing to do but to repeat with
the requisite [143/144] velocity and strength its perpendicular blows upon the air, and by virtue of
the structure of its wings the same blow both sustains and propels it.3

The truth of this explanation of the mechanical theory of flight may be tested
in various ways: In the first place it is quite visible to the eye. In many
birds flying straight to us, or straight from us, the effect of aërial
resistance in bending upwards the ends of the quill feathers is very conspicuous.
The flight of the common Rook affords an excellent example where the bird is
seen foreshortened. In Eagles the same effect is very marked — the wing tips forming
a sharp upward curve. I have seen it equally obvious in that splendid bird the
Gannet, or Solan Goose; and when we recollect the great weight which those few
quill feathers are thus seen sustaining, we begin to appreciate the degree in
which lightness, strength, and imperviousness to the passage of
[144/145] air are combined in this wonderful implement of flight.

But perhaps the simplest test of the action and reaction of the air and the
wing feathers in producing forward motion is an actual experiment. If we take
in the hand the stretched wing of a Heron, which has been dried in that position,
and strike it quickly downwards in the air, we shall find that it is very difficult
indeed to maintain the perpendicular direction of the stroke, requiring, in
fact, much force to do so; and that if we do not apply this force, the hand
is carried irresistibly forward, from the impetus in that direction which the
air communicates to the wing in its escape backwards from the blow. Another
test is one of reasoning and observation. If the explanation now given be correct,
it must follow that since no bird can flap its wings in any other direction
than the vertical — i.e., perpendicular to its own axis, (which is ordinarily
horizontal,) and, as this motion has been shown to produce necessarily a forward
motion, no bird can ever fly backwards. Accordingly no bird ever does
so — no man ever saw a bird, even for an instant, fly tail [145/146] foremost. A bird can, of course, allow itself to fall backwards by merely slowing
the action of its wings so as to allow its weight to overcome their sustaining
power; and this motion may sometimes give the appearance of flying backwards, — as
when a Swift drops backwards from the eaves of a house, or when a Humming Bird
allows itself to drop in like manner from out of the large tubular petals of
a flower. But this backward motion is due to the action of gravity, and not
to the action of the bird's wing. In short, it is falling downwards, not flying
backwards. Nay, more, if the theory of flight here given be correct, it must
equally follow that even standing still, which is the easiest of all things
to other animals, must be very difficult, if not altogether impossible, to a
bird when flying. This also is true in fact. To stand still in the air is not
indeed impossible to a flying bird, for reasons to be presently explained, but
it is one of the most difficult feats of flying [inanship??], a feat which
many birds, not otherwise clumsy, can never perform at all, and which is performed
only by special exertion, and generally for a very short time, by those [146/147] birds whose structure enables them to be adepts in their glorious art.

It cannot be too often repeated — because misconception on this point has been
the cardinal error in human attempts to navigate the air — that in all the beautiful
evolutions of birds upon the wing, it is weight, and not buoyancy, which makes
those evolutions possible. It supplies them, so to speak, with a store of Force
which is constant, inexhaustible, inherent in the very substance of themselves,
and entirely independent of any muscular exertion. All they have to do is to
give direction to that internal Force, by acting on the external Force of aërial
currents, through the contraction and expansion of the implements which have
been given them for that purpose. Those who have watched the flight of birds
with any care, must have observed that when once they have attained a certain
initial velocity and a certain elevation, by rapid and repeated strokes upon
the air, they are then able to fly with comparatively little exertion, and very
often to pursue their course for long distances without any flapping whatever
of the wings. [147/148] The contrast between the violent efforts required for the first acquisition
of the initial velocity, and the perfect ease with which flight is performed
after it has been acquired, is a contrast described by Virgil in lines of incomparable
beauty: —

Still more remarkable, as showing the power and the value of weight in flight,
is the fact that birds are able to resume rapid and easy motion not only as
the result of a previously — acquired momentum, but after"soaring" in an almost
perfectly stationary position. Nothing, for example, is more common than to
see Sea Gulls, and some large species of Hawks,"soaring" one moment, (that
is, all the forces bearing on the bird brought to an equilibrium, and all motion
brought consequently to nearly a perfect standstill,) and the next moment sailing
onwards in rapid and apparently effortless progression. Now, how is this effect
produced? If we only think of [148/149] it, the question ought rather to be, How is it ever prevented? The soaring
is a much more difficult thing to do than the going onwards. It cannot be done
at all in a perfectly still atmosphere. It can only be done when there is a
breeze of sufficient strength. Gravity is ceaselessly acting on the bird to
pull it downwards: and downwards it must go, unless there is a countervailing
Force to keep it up. This force is the force of the breeze striking against
the vanes of the wings. But in order to bring these two forces to nearly a perfect
balance, and so to"soar," the bird must expand or contract its wings exactly
to the right size, and hold them exactly at the right angle. The slightest alteration
in either of these adjustments produces instantly an upsetting of the balance,
and of course a resulting motion. The exact direction of that motion will depend
on the degree in which the wing is contracted, and the degree in which its angle
to the wind is changed. If the wing is very much contracted, and at the same
time held off from the wind, that motion will be steeply downwards. Accordingly
this is the action of a Hawk when it swoops upon its prey [149/150] from a great height above it. I have seen a Merlin dash down from a great distance
with its wings so closed as to seem almost wholly folded. The Gannet in diving
for fish does not close its wings at all, but turning them and the whole axis
of its body into the perpendicular, and thus allowing its great weight to act
without any counteraction, dashes itself into the sea with foam, But every variety
of forward motion is attained by different degrees of contraction and exposure,
according to the strength of the breeze with which the bird has to deal. The
limit of its velocity is the limit of its momentum, and the limit of its momentum
is the limit of its weight. The lightness of a bird is therefore a limit to
its velocity. The heavier a bird is, the greater is its possible velocity of
flight — because the greater is the store of force — or, to use the language of
modern physics, the greater is the quantity of"potential energy" — which, with
proper implements to act upon aerial resistance, it can always convert into
upward, or horizontal, or downward motion, according to its own management and
desires.

It will be at once seen from this view of the [150/151] forces concerned in flight, that the common explanation of Birds being assisted
by air — cells for the inhalation and storage of heated air, must not only be
erroneous, but founded on wholly false conceptions of the fundamental mechanical
principles on which flight depends. If a Bird could inhale enough warm air to
make it buoyant, its power of flight would be effectually destroyed. It would
become as light as a Balloon, and consequently as helpless. If, on the other
hand, it were merely to inflate itself with a small quantity of hot air insufficient
to produce buoyancy, but sufficient to increase its bulk, the only effect would
be to expose it to increased resistance in cleaving the air. It is true, indeed,
that the bones of Birds are made more hollow and lighter than the bones of Mammals,
because Birds, though requiring weight, must not have too much of it. It is
true, also, that the air must have access to these hollows, else they would
be unable to resist atmospheric pressure. But it is no part whatever of the
plan or intention of the structure of Birds, or of any part of that structure,
to afford balloon — space for heated air with a view to buoyancy. [151/152] And here, indeed, we open up a new branch of the same inquiry, showing, in
new aspects, how the universality and unchangeableness of all natural laws are
essential to the use of them as the instruments of Will; and how by being played
off against each other they are made to express every shade of thought, and
the nicest change of purpose. The movement of all flying animals in the air
is governed and determined by Forces of muscular power, and of aërial resistance
and elasticity being brought to bear upon the Force of Gravity, whereby, according
to. the universal laws of motion, a direction is given to the animal which is
the resultant, or compromise between all the Forces so employed. Weight, as
we have seen, is one of these Forces — absolutely essential to that result, and
no flying animal can ever for a moment of time be buoyant, or lighter than the
air in which it is designed to move. But it is obvious that, within certain
limits, the proportion in which these different Forces are balanced against
each other, admits of immense variety. The limits of variation can easily be
specified. Every flying animal must have muscular power great enough to work
its [152/153] own size of wing: that size of wing must be large enough to act upon a volume
of air sufficient to lift the animal's whole weight: lastly, and consequently,
the weight must not be too great, or dispersed over too large a bulk. . But
within these limits there is room for great varieties of adjustment, having
reference to corresponding varieties of purpose. To some birds the air is almost
their perpetual home — the only region in which they find their food — a region
which they never leave, whether in storm or sunshine, except during the hours
of darkness and the yearly days which are devoted to their nests. Other birds
are mainly terrestrial, and never betake themselves to flight except to escape
an enemy, or to follow the seasons and the sun. Between these extremes there
is every possible variety of habit. And all these have corresponding varieties
of structure. The birds which seek their food in the air have long and powerful
wings, and so nice an adjustment of their weight to that power and to that length,
that the faculty of self — command in them is perfect, and their power of direction
so accurate that they can pick up a flying gnat whilst passing through [153/154] the air at the rate of more than a hundred miles an hour. Such especially are
the powers of some species of the Swallow tribe, one of which, the common Swift,
is a creature whose wonderful and unceasing evolutions seem part of the happiness
of summer and of serene and lofty skies.4

There are other birds in which the wing has to be adapted to the double purpose
of swimming, or rather of diving, and of flight. In this case, a large area
of wing must be dispensed with, because it would be incapable of being worked
under water. Consequently in all diving birds the wings are reduced to the smallest
possible size which is consistent with retaining the power of flight at all; and in a few extreme Forms, the power of flight is sacrificed altogether,
and the wing is reduced to the size, and adapted to the function, of a powerful
fin. This is the condition of the Penguins. But in most genera of swimming Birds,
both purposes are combined, and the wing is just so far reduced in size and
stiffened in texture as to make it workable as a fin under water, whilst it
is still just large [154/155] enough to sustain the weight of the bird in flight.

And here again we have
a wonderful example of the skill with which inexorable mechanical laws are subordinated
to special purpose. It is a necessary consequence of the area of the wing being
so reduced, in proportion to the size of the bird, that great muscular power
must be used in working it, otherwise the Force of Gravity could not be overcome
at all. It is a farther consequence of this proportion of weight to working
power, that there must be great momentum and therefore great velocity of flight.
Accordingly this is the fact with all the oceanic diving Birds. They have vast
distances to go, following shoals of fish, and moving from their summer to their
winter haunts. They all fly with immense velocity., and the wing-strokes are
extremely rapid. But there is one quality which their flight does not possess — because
it is incompatible with their structure, and because it is not required by their
habits — they have no facility in evolutions, no delicate power of steering; they cannot stop with ease, nor can they resume their onward motion in a moment.
They do not want it: the trackless fields of ocean over which [155/156] they roam are broad, and there are no obstructions in the way. They fly in
straight lines, changing their direction only in long curves, and lighting in
the sea almost with a tumble and a splash. Their rising again is a work of great
effort, and generally they have to eke out the resisting power of their small
wings, not only by the most violent exertion, but by rising against the wind,
so as to collect its force as a help and addition to their own.

And now, again, we may see all these conditions changed where there is a change
in the purpose to be served. There is another large class of oceanic Birds whose
feeding ground is not under water, but on the surface of the sea. In this class
all those powers of flight which would be useless to the Divers, are absolutely
required, and are given in the highest perfection, by the enlistment of the
same mechanical laws under different conditions. In the Gulls, the Terns, the
Petrels, and in the Fulmars, with the Albatross as their typical Form, the mechanism
of flight is carried through an ascending scale, to the highest degrees of power,
both as respects endurance and facility of evolution. [156/157] The mechanical laws which are appealed to in all these modifications of structure
require adjustments of the finest kind, and some of them are so curious and
so beautiful that it is well worth following them a little further in detail.

There are two facts observable in all Birds of great and long — sustained powers
of flight: — the first is, that they are always provided with wings which are
rather long than broad, sometimes extremely narrow in proportion to their length;
the, second is, that the wings are always sharply pointed at the ends. Let us
look at the mechanical laws which absolutely require this structure for the
purpose of powerful flight, and to meet which it has accordingly been devised
and provided.

One law appealed to in making wings rather long than broad is simply the law
of leverage. But this law has to be applied under conditions of difficulty and
complexity, which are not apparent at first sight. The body to be lifted is
the very body that must exert the lifting power. The force of gravity which
has to be resisted may be said to be sitting side by side, occupying the same
particles of matter, with the vital force which is to give [157/158] it battle. Nay, more, the one is connected with the other in some mysterious
manner which we cannot trace or understand. A dead bird weighs as much as a
living one. Nothing which our scales can measure is lost when the"vital force"
is gone. It is The Great Imponderable. Nevertheless, vital forces of unusual
power are always coupled with unusual mass and volume — in the matter through
which they work. And so it is that a powerful bird must always also be comparatively
a heavy bird. And then it is to be remembered that the action of gravity is
constant and untiring. The vital force, on the contrary, however intense it
may be, is intermitting and capable of exhaustion. If, then, this force is to
be set against the force of gravity, it has much need of some implement through
which it may exert itself with mechanical advantage as regards the particular
purpose to be attained. Such an implement is the lever — and a long wing is nothing
but a long lever. The mechanical principle, or law, as is well known, is this, — that
a very small amount of motion, or motion through a very small space, at the
short end of a lever produces a [158/159] great amount of motion, or motion through a long space, at the opposite or
longer end. This action requires indeed a very intense force to be applied at
the shorter end, but it applies that force with immense advantage for the purpose
in view: because the motion which is transmitted to the end of along wing is
a motion acting at that point through along space, and is therefore equivalent
to a very heavy weight lifted through a short space at the end which is attached
to the body of the bird. Now this is precisely what is required for the purpose
of flight. The body of a bird does not require to be much lifted by each stroke
of the wing. It only requires to be sustained; and when more than this is needed — as
when a bird first rises from the ground, or from the sea, or when it ascends
rapidly in the air — greatly increased exertion — in many cases, very violent
exertion — is required 5 — And then it is to be remembered that [159/160] long wings economise the vital force in another way. When a strong current
of air strikes against the wings of a bird, the same sustaining effect is produced
as when the wing strikes against the air. Consequently birds with very long
wings have this great advantage, that with pre-acquired momentum, they can often
for a long time fly without flapping their wings at all. Under these circumstances,
a bird is sustained very much as a boy's kite is sustained in the air. The string
which the boy holds, and by which he pulls the kite downwards with a certain
force, performs for the kite the same offices which its own weight and balance
and momentum perform for the bird. The great long — winged oceanic birds often
appear to float rather than to fly. The stronger is the gale, their flight,
though less rapid, is all the more easy — so easy indeed as to appear buoyant
; because the blasts which strike against their wings are enough to sustain
the bird with comparatively little exertion of its own, except that of holding
described between the violent exertion required in first rising, and the perfect
ease of flight after this first momentum has been acquired, is a striking illustration
of the true mechanical principles of flight. [160/161] the wing vanes stretched and exposed at proper angles to the wind. And whenever
the onward force previously acquired by flapping, becomes at length exhausted,
and the ceaseless inexorable Force of Gravity is beginning to overcome it, the,
bird again rises by a few easy and gentle halfstrokes of the wing. Very often
the same effect is produced by allowing the force of gravity to act, and when
the downward momentum has brought the bird close to the ground or to the sea,
that force is again converted into an ascending impetus by a change in the angle
at which the wing is exposed to the wind. This is a constant action with all
the oceanic birds. Those who have seen the Albatross have described themselves
as never tired of watching its glorious and triumphant motion: —

Tranquil Tranquil its spirit seemed, and floated slow;
Even in its very motion there was rest."6

Rest — where there is nothing else at rest in the tremendous turmoil of its own
stormy seas! Sometimes for a whole hour together this splendid bird will sail
or wheel round a ship in every [161/162] possible variety of direction without requiring to give a single the Albatross
has kind of wing. Its — about fourteen tip — and almost stroke to its pinions.
Now, the extreme form of this wings are immensely long or fifteen feet from
tip to as narrow in proportion as a riband:7 Our common Gannet is an excellent,
though a more modified, example of the same kind of structure. On the other
hand, birds of short wings, though their flight is sometimes very fast, are
never able to sustain it very long. The muscular exertion they require is greater,
because it does not work to the same advantage. Most of the gallinaceous birds
(such as the common Fowl, Pheasants, Partridges, &c.) have wings of this
kind; and some of them never fly except to escape an enemy, or to change their
feeding ground. [162/163]

.
The second fact observable in reference to birds of easy and powerful flight — namely,
that their wings are all sharply pointed at the end — will lead us still further
into the niceties of adjustment which are so signally displayed in the machinery
of flight.

The feathers of a bird's wing have a natural threefold division, according
to the different wingbones to which they are attached. The quills which form
the end of the wing are called the 'Primaries: those which form the middle of
the vane are called the Secondaries; and those which are next the body of the
bird are called the Tertiaries. The motion of a bird's wing increases from its
minimum at the shoulder — joint to its maximum at the tip. The primary quills
which form the termination of the wing are those on which the chief burden of
flight is cast. Each feather has less and less weight to bear, .and less and
less force to exert in proportion as it lies nearer the body of the bird; and
there is nothing more beautiful in the structure of a wing than the perfect
gradation in strength and stiffness, as well as in modification of form, which
marks the series [163/164] from the first of the primary quills to the last and feeblest of the tertiaries.8
Now, the sharpness or roundness of a wing at the tip depends on the position
which is given to the longest primary quill. If the first or even the second
primary is the longest, and all that follow are considerably shorter, the wing
is necessarily a pointed wing, because the tip of a single quill forms the end;
but if the third or fourth primary quills are the longest, and the next again
on both sides are only a little shorter, the wing becomes a roundended wing.
Round — ended wings are also almost always open — ended — that is to say, the tips
of the quills do not touch each other, but leave interspaces at the end of the
wing, through which, of course, a good deal of air escapes. Since each single
quill is formed on the same principle as the whole wing — that is, with the
anterior margin stiff and the posterior margin yielding — this

WING OF A GOLDEN PLOVER. [Click on thumbnail for larger image.]

[164/165] escape is not useless for progression; but the air acts less favourably for
this purpose than when struck by a more compact set of feathers. The common
Rook and all the Crows are examples of this. The Peregrine Falcon, the common
Swallow, and all birds of very powerful flight have been provided with the sharp — pointed
structure.9

The object of this structure, and the mechanical laws to which it appeals,
will be apparent when we recollect what it is on which the propelling power,
as distinct from the sustaining power, of a bird's wing depends. It depends
on the reaction of the air escaping backwards — that is, in the direction
exactly opposite to that of the intended motion of the bird. Any air which escapes
from under the wing, in any other direction, will of course react with less
advantage upon that motion But from under a round wing a good deal of air must
necessarily escape along the rounded end — that is, in a direction at
right angles — to the line of intended flight. All the reaction produced by this [165/166] escape is a reaction which is useless for propulsion. Accordingly, in all birds
to whom great velocity of flight is essential, this structure, which is common
in other birds, is carefully avoided. The Hawks have been classified as "noble"
or "ignoble," according to the length and sharpness of their wings: those
which catch their prey by velocity of flight having been uniformly provided
with the long — pointed structure. The Sparrow — Hawk and the Merlin are excellent
examples of the difference. The Sparrow — Hawk, with its comparatively short and
blunt wings,' steals along the hedgerows and pounces on its prey by surprise;
seldom chasing it, except for a short distance, and when the victim is at a
disadvantage. And well do the smaller birds know this habit, and the limit of
his powers. Many of them chase and"chaff" the Sparrow — Hawk, when he is seen
flying in the open, perfectly aware that he cannot catch them by fast flying.
But they never play these tricks with the Merlin. This beautiful little Falcon
hunts the open ground, giving fair chase to its quarry by power and speed of
flight. The Merlin delights in flying at some of the fastest birds, such as

[166/167]
the Snipe. The longest and most beautiful trial of wingmanship I have ever
seen was the chase of a Merlin after a Snipe in one of the Hebrides. It lasted
as far as the eye could reach, and seemed to continue far out to sea. In the
Merlin, as in all the fastest Falcons, the second quill feather is the longest
in the wing; the others rapidly diminish; and the point of the wing looks as
sharp as a needle in the air.

There is yet one other power which it is absolutely necessary to some birds
that their wings should enable them to exert; and that is the power of standing
still, or remaining suspended in the air without any forward motion. One familiar
example of this is the common Kestrel, which, from the frequent exercise of
this power, is called in some counties the "Windhover." The mechanical principles
on which the machinery of flight is adapted to this purpose, are very simple.
No bird can exercise this power which is not provided with wings large enough,
long enough, and powerful enough to sustain its weight with ease, and without
violent exertion. Large wings can always be diminished at the pleasure of the
bird, by being [167/168] partially folded inwards; and this contraction of the area is constantly resorted
to. But a bird which has wings so small and scanty as to compel it to strike
them always at full stretch, and with great velocity in order to fly at all,
is incapable of standing still in the air. No man ever saw a Diver or a Duck
performing the evolution which the Kestrel may be seen performing every hour
over so many English fields. The cause of this is obvious if we refer to the
principles which have already been explained. We have seen that the perpendicular
stroke of a bird's wing has the double effect of both propelling and sustaining.
The reaction from such a stroke brings two different forces to bear upon the
bird — one whose direction is upwards, and another whose direction is forwards.
How can these two effects be separated from each other? How can the wing be
so moved as to keep up just enough of the sustaining force without allowing
the propelling force to come into play? The answer to this, although it in
volves some very complicated laws connected with what mechanicians call the
"parallelogram of forces," is practically a simple one. It can only [169] be done by shortening the stroke, and altering the perpendicularity of its
direction. Of course, if a bird, by altering the axis of its own body, can direct
its wing — stroke in some degree forwards , it will have the effect of stopping
instead of promoting progression. But in order to do this, it must have a superabundance
of sustaining force, because some of this force is sacrificed when the stroke
is off the perpendicular. Hence it follows that birds so heavy as to require
the whole action of their wings to sustain them at all, can never afford this
sacrifice of the sustaining force, and except for the purpose of arresting their
flight, can never strike except directly downwards, that is, directly against
the opposing force of gravity. But birds with superabundant sustaining power,
and long sharp wings, have nothing to do but to diminish the length of stroke,
and direct it off the perpendicular at such an angle as will bring all the forces
bearing upon their body to — an exact balance, and they will then remain stationary
at a fixed point in the air.10
[169/170] They are greatly assisted in this beautiful evolution by an adverse current
of air; and it will always be observed that the Kestrel, when hovering, turns
his head to wind, and hangs his whole body at a greater or less angle
to the plane of the horizon. When there is no wind, or very little, the sustaining
force is kept up by a short rapid action of the pinions, and the long tail is
spread out like a fan to assist in stopping any tendency to onward motion. When
there is a strong breeze, no flapping is required at all — the force of the wind
supplying the whole force necessary to counteract the force of gravity; and
in proportion to the increasing strength of the wind, the amount of vane which
must be exposed to it becomes less and less. I have seen a Kestrel stand suspended
in a half gale with the wings folded close to the body, and with no visible
muscular motion whatever. And so nice is the adjustment of position which is
requisite to produce this exact balance of all the forces bearing on the bird,
that the change in that position which again instantly results in a forward
motion is very often almost insensible to the eye. It is generally a [170/171] slight expansion of the wings, and a very slight change in the axis of the
body.

And here it may be observed that the tails of birds have not, as is often supposed,
any function analogous to the rudder of a ship. Birds which have lost the tail
are not thereby rendered incapable of turning. If the steering function had
been assigned to birds' tails, the vane of the tail must have been set, not,
as it is, horizontally, but perpendicularly to the line of flight. But a bird's
tail has in flight no lateral motion whatever. It does, indeed, materially assist
the bird in turning, because it serves to stop the way of a bird when
it rises or turns in the air to take a new direction. It contributes also largely
to the general balance of the body, which in itself is an important element
in the facility of flight. Accordingly, almost all birds which depend on great
ease of evolution in flight — or on the power of stopping suddenly, have largely
developed tails. This is the case with all the birds of prey — with the Kestrel
in a conspicuous degree. But there are some exceptions which show that great
powers of flight are not [171/172] always dependent on the possession of a large tail — as, for example, the Swift.

Another explanation has been given of the means by which birds are able to
turn in flight, which is a curious example how preconceived theories founded
on false analogies will vitiate our observation of the commonest facts in nature.
I do not know of any modern work which gives any account of the theory of flight,
which is even tolerably correct. But in most points an admirable account is
to be found in the celebrated work of Borelli," De Motu Anilrialium." On the
question, however, of steerage in flight, he gives a solution which the most
ordinary observation is sufficient to contradict. Borelli is quite aware that
the tail in birds has no such function as that which is usually assigned to
it, and he points out the true theoretical objection to the possibility of its
having any guiding power — viz., its horizontal position, and its immobility in
the lateral direction. But the theory which he himself propounds is equally
erroneous. It is this, — that birds deflect their course to the right or to the
left, as rowers turn a row — boat — by striking more quickly and [172/173] more strongly with one wing than with the other.11

To this theory there are two objections — first, that as a matter of fact birds
can turn, and do turn, even to the extent of describing complete circles in
the air, without any flapping either of one wing or the other; and secondly,
that when birds do flap and turn at the same time, not the slightest difference
in time between the two wingstrokes can ever be detected. The beats of a bird's
two wings are always exactly synchronous. But the first of these two objections
is of itself quite sufficient to disprove the theory. No man can have watched
even for a moment the flight of the common Swallow, and especially the flight
of the Swift, without seeing it perform complete gyrations in the air without
any strokes of either wing. The only change which can ever be detected by the
eye is a [173/174] slight elevation of one side of the whole body, and a slight depression of
the other. The depression is always on that side towards which the bird is turning.
On the opposite side, that from which the bird is turning, there is of course
a corresponding elevation. Sometimes this is very obvious; but in general it
is so slight as to require close observation to detect it. In the Albatross,
when sweeping round, the wings are often pointed in a direction nearly perpendicular
to the sea.12 The effect of this, of course, is to expose the two vanes at different
angles to the aerial currents — and it must be remembered that in flight the
balance of all the forces employed is so extremely fine that the most minute
alteration in the degree in which they bear upon each other, will produce an
immense change in the [174/175] result. It is not surprising, therefore, that the muscular movements which
serve to turn the axis of a flying bird from one direction to another, are very
often so extremely minute as generally altogether to elude the sight. But in
general terms, it may be said that a bird turns in flying essentially on the
same principle as that on which a man turns in walking. It is done in both cases
by change in the direction of muscular pressure upon a resisting medium. By
an exquisite combination of different laws, and by mechanical contrivance in
the adjustment of them, it has been given to a bird to find in the thin and
yielding air a medium of resistance against which its own muscular force may
act, as firm and as effective as that which Man finds in the solid earth.

The Humming — Birds are perhaps the most remarkable examples in the world of
the machinery of flight. rThe power of poising themselves in the air, — remaining
absolutely stationary whilst they search the blossoms for insects, — is a power
essential to their life.? It is a power accordingly which is enjoyed by them
in the highest perfection. When they intend progressive flight, it is effected [176] with such velocity as to elude the eye. The action of the wing in all these
cases is far too rapid to enable the observer to detect the exact difference
between that kind of motion which keeps the bird at absolute rest in the air,
and that which carries it along with such immense velocity. But there can be
no doubt that the change is one from a short quick stroke delivered obliquely
forward, to a full stroke, more slow, but delivered perpendicularly. This corresponds
with the account given by that most accurate ornithological observer, Mr Gould.
He, says:"When poised before any object, this action of the wing is so rapidly
performed that it is impossible for the eye to follow each stroke, and a hazy
semicircle of indistinctness on each side of the bird is all that is perceptible."
There is another fact mentioned by those who have watched their movements most
closely which corresponds with the explanation already given — viz., the fact
that the axis of the Humming Bird's body when hovering is always highly inclined, so much so as to appear almost perpendicular in the air. In other words
the wingstroke, instead of being delivered perpendicularly [176/177] downwards, which would infallibly carry the body onwards, is delivered at such
an angle forwards as to bring to an exact balance the upward, the downward,
and the forward forces which bear upon the body of the bird. Mr Darwin says,
"when hovering by a flower, the tail is constantly shut and expanded like a
fan, the body being heft in a nearly vertical position." Mr Wallace,
another accurate observer, describes the Humming Birds as"balancing themselves
vertically in the air."

These are a few, and a few only, of the adjustments required in order to the
giving of the power of flight; — adjustments of organic growth to intensity of
vital force — of external structure to external work — of shape in each separate
feather to definite shape in the series as a whole — of material to resistance — of
mass and form to required velocities; adjustments, in short, of law to law,
of force to force, and of all to Purpose. So many are these contrivances, so
various, so fine, so intricate, that a volume might be written without exhausting
the beauty of the method in which this one mechanical problem has been solved.
it is by knowledge of unchanging laws that these victories over them [177/178] seem to be achieved! yet not by knowledge only, except as the guide of Power.
For here, as everywhere else in Nature, we see the same mysterious need of conforming
to imperative conditions, side by side with absolute control over the forces
through which this conformity is secured When any given purpose cannot be attained
without the violation of some law, unless by some new power, and some new machinery — the
requisite power and mechanism are evolved — generally out of old materials, and
by modifications of pre-existing forms. There can be no better example of this
than a wing — feather. It is a production wholly unlike any other animal growth — an
implement specially formed to combine strength with lightness, elasticity, and
imperviousness to air. Again, the bones of a bird's wing are the bones of the
Mammalian arm and hand, specially modified to support the feathers. The same
purpose is effected by other means in connexion with precisely the same bones
in the flying Mammalia — the Bats. In these animals the finger — bones instead of
being compressed or soldered together to support feathers, are separated, attenuated,
and greatly [178/179] lengthened to afford attachment to a web or flying membrane which is stretched between
them. In other ages of the world there were also flying Lizards. But in all
these cases the mechanical principle is the same, and there has been the same
ingenious adaptation of material and of force to the universal laws of motion.

On the earth and on the sea Man has attained to powers of locomotion with which,
in strength, endurance, and in velocity, no animal movement can compare. But
the air is an element on which he cannot travel — an ocean which he cannot navigate.
The birds of heaven are still his envy, and on the paths they tread he cannot
follow. As yet! for it is not certain that this exclusion is to be perpetual.
His failure has resulted quite as much from his ignorance of natural laws, as
from his inability to meet the conditions which they demand. All attempts to
guide bodies buoyant in the air must be fruitless. Balloons are mere toys. No
flying animal has ever been formed on the principle of buoyancy. Birds, and
Bats, and Dragons, have been all immensely heavier than the air, and their weight
is one of the forces most [179/180] essential to their flight. Yet there is a real impediment in the way of Man
navigating the air — and that is the excessive weight of the only great mechanical
moving powers hitherto placed at his disposal. When Science shall have discovered
some moving power greatly lighter than any we yet know, in all probability the
problem will be solved. But of one thing we may be sure — that if Man is ever
destined to navigate the air, it will be in machines formed in strict obedience
to the mechanical laws which have been employed by the Creator for the same
purpose in flying animals.13